It was impossible to independently verify the immediate impact of the strike, but Jose Genivaldo da Silva, a National Oil Workers director, said that at least 90 percent of Petroleo Brasileiro SA's more than 40,000 employees walked off their jobs on Thursday.

Petrobras officials confirmed a strike and said in a statement that the company has taken "all the measures needed to guarantee operations and the supply of products to the market." The statement did not provide further details.

Union director Silva said Petrobras contingency teams comprised of supervisors and managers were maintaining the company's operations at the refineries, offshore platforms and terminals where workers were on strike.

Workers are demanding the suspension of the upcoming auction of the Libra field under new production-sharing agreements with foreign companies that will partner with Petrobras.

Located off the coast of Rio de Janeiro in the Santos Basin, Libra is a pre-salt reserve believed to hold up to 12 million barrels of recoverable oil.

Over the past several years, Brazil has discovered billions of barrels of oil in offshore reserves, mostly in deep, pre-salt fields off its southeast coast.

Most of these fields lie more than a mile below the ocean's surface and under another 2.5 miles of earth and salt.

Silva said the strike was also part of the union's wage campaign.

The union is demanding an 11.6 percent wage increase while Petrobras said it was willing to increase salaries by 7.7 percent.